Technologies like microelectronics, micromechanics and biotechnology have created a high demand in industry for structuring and probing specimens within the nanometer scale. On such a small scale, probing or structuring is often done with electron beams which are generated and focussed in electron beam devices like electron microscopes or electron beam pattern generators. Electrons beams offer superior spatial resolution compared to e.g. photon beams due to their short wave lengths at a comparable particle energy.
While electron beam devices can meet high spatial resolution requirements they are often too slow to deliver the throughput needed in large scale manufacturing. To overcome the throughput limitations, electron beam devices with multiple beams have been proposed with various designs. In U.S. Pat. No. 6,145,438 a multiple electron beam device with microcolumns is proposed. Each microcolumn is able to generate an electron beam and direct it towards a common specimen with high spatial resolution. Usually microcolumns comprise electron beam optical components for focussing and deflecting the electron beams individually. Such a design allows for a high flexibility because each electron beam can be operated independently. However, microcolumns have diameters of at least 1 to 2 cm, which makes it difficult to generate electron beams with a density of more than one or two electron beams per square centimeter.
Electron multiple beam devices with higher electron beam density usually rely on arrays of field emission cathodes where the field emission cathodes are integrated onto a substrate. Such field emission cathode arrays are fabricated by using micromechanical or microelectronic fabrication techniques. They were first proposed by C. A. Spindt (Journal of Appl. Physics, Vol 39 (1968) No.7, p. 3504-3505). Field emission cathode arrays usually comprise an array of emitter tips and an array of extracting electrodes with extracting electrode and emitter tip facing each other one to one. Due to the sharp apices of the emitter tips, and due to the short distances between emitter tip and extracting electrode it is possible to generate an extremely high electric field at the apices with moderate voltages. As the electric field at the surface of an apex rises above say 107 V/cm, electrons in the emitter tip are able to tunnel through the surface potential barrier of the apex to be emitted into free space. This fact is used to generate primary electron beams where the electron beam currents are controlled by the voltages between emitter tip and facing extracting electrode.
Field emission cathode arrays have since evolved to advanced devices with various designs and features. It is now possible to integrate arrays of field emission cathodes with a pitch of a few micrometers or smaller onto a substrate. With such technology it is possible to integrate hundreds, thousands or even millions of field emission cathodes onto a substrate of the size of a thumbnail. Such high integration density however makes it difficult to control directions, focus lengths and electron beam currents of the primary electron beams individually.
To provide individual control of the final focus lengths of multiple electron beams of highly integrated field emission cathode arrays, it has been proposed to integrate an array of gate electrodes on the field emission cathode array with gate electrode and field emission cathode facing each other one to one. Such field emission cathode arrays are disclosed, e.g., in U.S. Pat. No. 5,929,557. The voltage of each gate electrode can be controlled individually to change or adjust shape or direction of each primary electron beam individually. In particular, the voltages of each gate electrode can be used to change or adjust the final focus lengths of the final foci of the primary electron beams individually, the term “final focus” thereby throughout this application refers to the focus of a primary electron beam which is meant to probe or structure the surface of a specimen. Shape, position and length of the final focus of a focussed primary electron beam is of particular importance since it defines at what position and spatial resolution the surface of a non-transparent specimen is probed or structured.
For an electron beam device with a single primary electron beam, positioning of the primary electron beam, adjustment of the focal length and control of the electron beam current is straightforward and can be carried out by hand. E.g., directing the primary electron beam to the desired primary electron beam position can be done by scanning a surface region of interest of the specimen and observing the image created by the secondary particles. The scan is usually repeated with various operational parameters of the magnetic or electrostatic lenses until the image of the structured upper surface shows best spatial resolution.
For an electron multiple beam device, the procedure of finding the primary electron beam position and the determining the beam currents is more complicated for several reasons. First, the primary electron beam position with respect to the specimen has to be known not only for one primary electron beam but for many primary electron beams. Second, the positions of the field emission cathodes in a field emission cathode array in practice show deviations from specified positions that limit the precision for probing or structuring a specimen. Third, when adjusting thousands or even millions of electron beams it is mandatory to adjust the primary electron beams in parallel to save time. However it is difficult to adjust the primary electron beams in parallel until the images created by each primary electron beam show best resolution. Fourth, the detection of secondary particles for the purpose of measuring the primary electron beam positions in parallel is hampered by the confusion of not knowing which secondary particle originates from what primary electron beam. Fifth, the measurement of the primary electron beam positions and the final focus lengths takes some time, during which the non-transparent specimen may charge up in the region where the measurements are being done. Such charge-up can destroy or damage the specimen. And finally, even with the same operational parameters for each electron beam source, the currents of the primary electron beams may vary considerably. This can cause inhomogeneities on a structured surface of a non-transparent specimen which are beyond acceptable specifications.